A multi-emitter laser diode produces a plurality of optical beams, one from each emitter. A common method of delivering the laser diode optical output to an intended application includes coupling the optical beams into a bundle of transport optical fibers. The input ends of the transport fibers are aligned to the laser diode emitters. Coupling optics are placed between the emitters and the transport fiber input ends to properly focus the laser diode output beams into the array of transport fibers. The laser diode, coupling optics and transport fiber input ends are enclosed inside a sealed fiber array package to prevent contamination of the laser diode emitters, coupling optics and transport fiber input ends.
It is desirous to monitor the operating parameters of the laser diode, such as output power, wavelength, pulse width, etc., during operation. The measured operating parameters are used to maximize the operating performance of the laser diode and/or the intended application.
One method of monitoring the laser diode output is to place a beam sampling device, such as a beam splitter, between the laser diode emitters and the input ends of the transport fibers. The sampling device directs a portion of the output beams into a detector. The use of such sampling devices, however, has several drawbacks. The distance between the laser diode and the coupling optics is very small. It is difficult to place a beam sampling device in such a small space. It is also mechanically difficult to place a beam sampling device between the coupling optics and the transport fibers that would maintain proper alignment, be reasonably uncomplicated and not take up excessive space. Further, a sampling device disposed between the laser diode and transport fibers would divert optical output that otherwise would be focused into the transport fibers, thus reducing the intensity of the optical output delivered to the intended application. Moreover, the sampling device would be an additional optical element along the optical path from the laser diode to the intended application that would be susceptible to misalignment and contamination. Lastly, such a sampling device must sample a portion of each beam emitted from the diode such that the energy directed to the detector properly exemplifies the overall output characteristics of all of the output beams from the laser diode.
Another method of monitoring the laser diode output is to directly monitor the light transported by the transport fiber. The light exiting the transport fiber can be sampled using a beam splitter, and the remaining light focused back into another transport fiber for delivery to the intended application. However, this method also diverts optical output that otherwise would be delivered to the intended application. Further, this method involves including additional optical elements in the laser diode delivery system, which are susceptible to contamination and misalignment.
One solution to simplify laser diode performance monitoring is described in U.S. Pat. No. 5,504,762, issued to Hutchison. This patent illustrates the well known technique of placing a cylindrical fiber lens lengthwise across the emitters to collimate/focus each of the laser diode beams into the transport fibers. Stray radiation is emitted out either end of the fiber lens along a stray radiation emission path, generally perpendicular to the emitter region emission path. A detector is positioned along the stray radiation emission path to monitor the performance of the laser diode.
There are, however, several disadvantages to the Hutchison configuration. First, the technique described in this patent does not necessarily measure the laser diode output with accuracy. Several factors can cause this stray radiation exiting the fiber lens to change relative to the overall laser diode output. For example, it can be envisioned that certain misalignments of the optical elements could vary the intensity, direction or divergence of the stray radiation exiting the fiber lens end while not introducing a corresponding change in the laser diode output, and/or the output energy coupled into the transport fibers. In addition, selective contamination of the coupling optics could significantly affect the intensity of scattered light while not introducing a corresponding change of laser diode power intensity or coupling efficiency. Further, the detector has to be placed adjacent the fiber lens, most likely inside the sealed package, to properly capture the diverging stray radiation emanating from the fiber lens. Therefore, the detector cannot be serviced or replaced without breaking the seal on the protective packaging. Moreover, optical filters and diagnostic tools cannot be introduced into the light before the light enters the detector. In addition, detector alignment to the stray radiation path would be critical. Any misalignment of the detector, either before or after fabrication of the laser diode package, would result in significant changes in the signal output of the detector. Lastly, sealed lead connections through the packaging walls are necessitated to extract the electrical signals from the detector out of the sealed protective packaging.
There is a need for a detection system that simply and accurately measures the output characteristics of a laser diode during operation. This detection system should not divert any light that otherwise would be focused into the transport fibers, and should minimize detection error due to optical element misalignment or contamination. In addition, it is preferable that such a detection system include a detector that is accessible without intruding within the sealed protective packaging surrounding the laser diode and coupling optics.